Diffusion-Assisted Formation Mechanism of Molecular Break Junctions: A First-Principles Study of Benzenethiolate on Au(111)

Density-functional theory (DFT) calculations have been used to obtain the binding energies of benzenethiolate on Au(111) under applied stress. The binding energetics to ideal Au(111), as well as to Au adatoms and small 2−3 adatom islands, have been calculated as functions of the normal displacement applied to the free end of the molecule. We find that binding to adatoms and adatom islands is energetically preferred over breaking the Au−S bond for sufficiently large displacements. On the basis of these results, we propose a diffusion-assisted mechanism of pyramidal break junction formation on Au(111) that involves creation and capture of surface adatoms, as well as surface or bulk vacancies. Quantitative estimates of stretching rates that are compatible with the required diffusion times are given on the basis of random walk theory.